1. Field of the Invention
The present invention relates to a wafer cleaning apparatus for cleaning wafers, which are processed by various processing steps such as plating and chemical mechanical polishing.
2. Description of the Related Art
When metallic materials such as copper attach themselves to the surfaces and edges of semiconductor wafers electro-chemically as metallic copper, they become stable and cannot be removed by washing with pure water. Copper adhering in such a manner will diffuse into silicon wafers during heat treatment, and causes problems in device performances.
On the other hand, a copper seed layer is currently formed as a pre-treatment for copper plating by sputtering copper (or by CVD) on the silicon wafer, and the trend is to apply the layer over the entire front surface of the wafer, because it is a more efficient use of the area. That is, as shown in FIG. 1, a barrier layer 80 is formed on the front surface of the wafer W extending to its edge section E, a Cu seed layer 83 is formed thereon, and a plating layer 85 is formed on the seed layer 83.
However, when a Cu seed layer 83 of 100 nm, for example, is formed by sputtering on the entire front surface of the wafer W, a sputtered Cu layer is formed not only on the front surface of the wafer but a thin sputtered Cu layer is formed also on the edge section E of the wafer as shown in FIG. 2. On the other hand, the formation of the plating layer 85 on the wafer W is carried out by sealing the outer peripheral section of the wafer W so as to prevent the plating layer from extending to the back surface of the wafer W. Therefore, the plating layer 85 can be formed only on the front surface of the wafer W not extending to the edge thereof, as shown in FIG. 2. For this reason, a portion of the Cu seed layer 83 remains on the edge section E and in the vicinity thereof as a thin layer. The residual Cu layer in such places can detach and be removed from the wafer during transport or processing of plated wafers or chemically mechanically polished (CMP) wafers, thereby leading to a high probability of causing Cu cross-contamination.
Also, it is difficult to prevent Cu from adhering to the back surface and edge of the wafer even if the edge and the back surface are protected by sealing the outer periphery thereof.